The present invention relates to a process for the preparation of acrolein and/or acrylic acid from propane, in which, in a first reaction zone, the propane is subjected to a partial oxydehydrogenation with molecular oxygen under homogeneous and/or heterogeneous catalysis to give propene, and the propene-containing product gas mixture formed in the first reaction zone is fed, without separating off a component of the product gas mixture, into at least one further reaction zone and, in this at least one further reaction zone, the propene contained in the product gas mixture of the first reaction zone, together with all components of the product gas mixture of the first reaction zone, is subjected to a gas-phase catalytic oxidation to acrolein and/or acrylic acid, acrolein and/or acrylic acid and water contained in the product gas mixture of the gas-phase catalytic oxidation are separated from said mixture, and the unconverted propane contained in the remaining residual gas stream is recycled as a component of the residual gas stream to the first reaction zone.
Acrolein and acrylic acid are important intermediates which are used, for example, in the preparation of active ingredients and polymers.
The process predominantly used at present on an industrial scale for the production of acrolein and/or acrylic acid is the gas-phase catalytic oxidation of propene (e.g. EP-A 575 897), the propene predominantly being produced as a byproduct of ethylene production by steam cracking of naphtha.
Since the other fields of use of propene, for example the preparation of polypropylene, are constantly expanding, it would be advantageous to have a competitive process for the preparation of acrolein and/or acrylic acid which can be used on an industrial scale and whose raw material base is not propene but, for example, the propane occurring naturally in large quantities as the component of natural gas.
U.S. Pat. No. 3,798,283 discloses that propane can be homogeneously oxydehydrogenated to propene in the presence of molecular oxygen at elevated temperatures. Suitable oxygen sources are both pure oxygen and mixtures of oxygen and inert gas.
DE-A 20 58 054 and DE-A 1 95 30 454 disclose that the oxydehydrogenation of propane to propene can also be carried out under heterogeneous catalysis.
U.S. Pat. No. 3,161,670, EP-A 117 446 and DE-A 33 13 573 relate to processes for the preparation of acrolein and/or acrylic acid in which propane is first subjected to dehydrogenation under heterogeneous catalysis in the absence of oxygen to give propene.
The propene-containing product mixture is then subjected to a gas-phase oxidation under heterogeneous catalysis. However, the disadvantage of this procedure is that the catalyst required for the nonoxidative dehydrogenation of propane is relatively rapidly deactivated by carbon deposits and must therefore be frequently regenerated. A further disadvantage of this procedure is the hydrogen formation associated with the nonoxidative propane dehydrogenation.
It is true that DE-A 33 13 573 mentions the possibility in principle of coupling oxidative dehydrogenation of propane to propene with a subsequent propene oxidation under heterogeneous catalysis. However, it contains no further information on carrying out such a process.
EP-A 293 224, U.S. Pat. No. 5,198,578 and U.S. Pat. No. 5,183,936 state that a high N2 content in the diluent gas of the catalytic gas-phase oxidation of propene to acrolein and/or acrylic acid is disadvantageous. EP-A 293 224 furthermore suggests combining the oxidative dehydrogenation of propane to propene and the catalytic gas-phase oxidation of propene with one another for the preparation of acrolein and/or acrylic acid.
In Catalysis Today 13 (1992), 673 to 678, Moro-oka et al. combine a homogeneous oxidative dehydrogenation of propane to propene with a subsequent oxidation of the dehydrogenation product mixture under heterogeneous catalysis to acrolein and/or acrylic acid. The corresponding combination of processes is recommended by Moro-oka et al. in Applied Catalysis 70 (2) (1991), 175 to 187. In line with the recommendation of EP-A 293 224, of U.S. Pat. No. 5,198,578 and of U.S. Pat. No. 5,183,936, Moro-oka et al. use either pure molecular oxygen or air depleted with respect to nitrogen in all cases as an oxygen source for the oxydehydrogenation stage. CN-A 1105352 adopts the same procedure.
Although WO 97/36849 does not rule out the direct use of air as an oxygen source for a catalytic oxidative dehydrogenation of propane to propene in combination with a subsequent propene oxidation to acrolein and/or acrylic acid, for limiting the N2 content in the subsequent propene oxidation state it envisages only the possibility of using, as an oxygen source in the oxydehydrogenation stage, a gas mixture which contains at least 90 mol % of oxygen. This applies in particular when recycling of the reaction gas mixture containing unconverted propane, freed from acrolein and/or acrylic acid and obtained from the catalytic gas-phase oxidation stage to the oxydehydrogenation stage is taken into account, since such recycling would indeed result in an increase in the nitrogen content in the gas-phase oxidation. Moreover, WO 97/36849 envisages merely a purge of recycled gas and not removal of components from the recycled gas for the case of a continuous procedure with gas recycling for suppressing an undesired increase in the concentration of disadvantageous components of the reaction gas mixture.
For cost-efficiency reasons, essentially only air is suitable as a starting material for the molecular oxygen source for industrial gas-phase oxidations.
Against this background, the abovementioned procedures are disadvantageous in that, owing to the similarity of O2 and N2 starting from air, the sole measure of a prior nitrogen/oxygen separation for the preparation of pure oxygen or of air depleted with respect to nitrogen for the limiting the nitrogen content in a subsequent oxidation of the propene contained in the dehydrogenation product mixture is very energy-consuming.
It is an object of the present invention to provide a process for the preparation of acrolein and/or acrylic acid from propane, in which, in a first reaction zone, the propane is subjected to a partial oxydehydrogenation with molecular oxygen under homogeneous and/or heterogeneous catalysis to give propene, and the propene-containing product gas mixture formed in the first reaction zone is fed, without separating off a component of the product gas mixture, into at least one further reaction zone and, in this at least one further reaction zone, the propene contained in the product gas mixture of the first reaction zone, together with all components of the product gas mixture of the first reaction zone, is subjected to a gas-phase catalytic oxidation to acrolein and/or acrylic acid, acrolein and/or acrylic acid and water contained in the product gas mixture of the gas-phase catalytic oxidation are separated from said mixture, and the unconverted propane contained in the remaining residual gas stream is recycled as a component of the residual gas stream to the first reaction zone, and in which the limitation of the nitrogen content in the propene oxidation stage is effected in a less energy-consuming manner than in the prior art.
We have found that this object is achieved by a process for the preparation of acrolein and/or acrylic acid from propane, in which, in a first reaction zone, the propane is subjected to a partial oxydehydrogenation with molecular oxygen under homogeneous and/or heterogeneous catalysis to give propene, and the propene-containing product gas mixture formed in the reaction zone is fed, without separating off a component of the product gas mixture, into at least one further reaction zone and, in this at least one further reaction zone, the propene contained in the product gas mixture of the first reaction zone, together with all components of the product gas mixture of the first reaction zone, is subjected to a gas-phase catalytic oxidation to acrolein and/or acrylic acid, acrolein and/or acrylic acid and water contained in the product gas mixture of the gas-phase catalytic oxidation are separated from said mixture, and the unconverted propane contained in the remaining residual gas stream is recycled as a component of the residual gas stream to the first reaction zone, wherein the molecular oxygen required in the first reaction zone and differing from recycled oxygen is added in the form of air to the reaction gas starting mixture fed to the first reaction zone and, before the recycling of the residual gas stream to the first reaction zone, at least a part of the atmospheric nitrogen contained in the residual gas stream is separated from the residual gas stream.